This invention relates to the valve control apparatuses and, in particular, to valve control apparatuses for diesel engine compression release brakes.
Compression release brakes are used to slow diesel powered vehicles such as large tractor trailer units. These brakes work by releasing compressed gases from each cylinder near top dead center of each compression stroke. This removes the rebound effect whereby the compressed gases would tend to drive the piston downwardly and thereby counter the braking effect otherwise created when the pistons compress gases during the compression stroke. Engine brakes are normally operated when a vehicle is coasting downhill and the fuel supply to the engine has been cut off. Wear on the wheel brakes is reduced since an engine brake significantly reduces the braking contribution required from the wheel brakes.
At least one exhaust valve on each cylinder is cracked open just before top dead center of each compression stroke when the brake is operational. Some mechanism must be provided, therefore, to open each exhaust valve twice during each engine cycle. The normal exhaust valve opening occurs during the exhaust stroke when the piston is moving upwardly towards the cylinder head. The second exhaust valve opening occurs during braking operation near the top dead center position at the end of the compression stroke. Various mechanisms have been devised to selectively crack open each exhaust valve the second time during each engine cycle. In many engines, for example, a fuel injector mechanism is used to crack open each exhaust valve at the required time. However such a mechanism is not available, nor suitable for all types of engines. Accordingly, alternative mechanisms have been devised.
U.S. Pat. Nos. 5,537,976 and 5,680,841, both to Hu, disclose the concept of providing a hydraulic linkage between the camshaft and the exhaust valves. The camshaft has two lobes for each exhaust valve, a first of the lobes opening each exhaust valve normally during the exhaust stroke. The system employs a cam follower hydraulically connected to each exhaust valve. Clearance between the cam follower and the camshaft is effectively changed whereby a second cam lobe, smaller than the first lobe, actuates the valve during brake operation.
One problem with such prior art engine brakes is that the normal operation of the exhaust valve is affected during brake operation. Clearance between the cam follower and camshaft is effectively reduced during brake operation. This means that the first lobe on the camshaft opens the exhaust valve further than normal for the exhaust stroke during exhaust brake operation. In some cases it is necessary to provide recesses in the pistons so that the exhaust valves do not strike the pistons when the brake is operational. These recesses, and the abnormally extended exhaust valves, interfere with optimal engine design from the point of view of other considerations such as emission controls.
Another problem with such prior art engine brakes is that the exhaust valve overlap at top dead center is increased during brake operation. This means that exhaust gas energy is lost from the exhaust manifold to the inlet stroke of the cylinder. Recovering the lost energy would be beneficial in order to drive the turbocharger to supercharge the compression stroke.
It is an object of the invention to provide an improved valve control apparatus which overcomes the disadvantages associated with the prior art.
It is also an object of the invention to provide an improved valve control apparatus which allows a camshaft to selectively open each exhaust valve near top dead center of each compression stroke, for engine braking purposes, without interfering with normal maximum lift and closing of each exhaust valve on each exhaust stroke.
Is a further object of the invention to provide an improved valve control apparatus which is rugged and economical in construction and reliable during operation.